The present invention relates to gas treatment or separation apparatus of the kind that includes a vessel containing a bed of particulate material. Such vessels are used throughout the chemical industry to perform a wide range of different operations. For example, the bed may comprise particles of catalyst and the bed may therefore be used to perform a catalytic reaction amongst components of the gas or gas mixture entering the vessel. In another example, the bed may comprise particles of an adsorbent adapted selectively to remove one or more components of the gas or gas mixture. Such vessels find a wide spread use in gas purification and separation processes. A particular example of such a gas separation process is pressure swing adsorption.
In a pressure swing adsorption process, an incoming gas mixture at a relatively high pressure is passed through a bed of molecular sieve adsorbent which selectively absorbs one or more components from the gas mixture, and a resulting non adsorbed product gas is withdrawn from the bed at a pressure only a little below the pressure of the incoming gas mixture. It will be appreciated that during the adsorption, the molecular sieve will become fully loaded with the selectively adsorbed components with the result that unless the adsorption is ended, the product gas will become progressively more contaminated with these components. At the end of the adsorption, therefore, the bed is regenerated by having the pressure therein reduced. The pressure reduction causes the previously adsorbed components to be desorbed and the resulting desorbed gas is conducted out of the bed. The bed is then ready to resume its role in separating the incoming gas mixture. Typically, two or more such beds are used in appropriate phase relationship with one another so as to minimise or eliminate periods of time in which the incoming gas mixture is not being separated in one vessel or another. Typically, in such pressure swing adsorption processes the incoming gas is compressed to a pressure in the range of 2 to 10 atmospheres absolute and each adsorption step takes at least 3/4 of a minute to perform. When it is desired to separate a nitrogen product from air, carbon molecular sieve is typically used as the molecular sieve, whereas when it is desired to separate an oxygen product from air, a zeolite molecular sieve is used. In order to improve upon commercial processes for separating oxygen from air, it has been proposed to reduce the duration of each adsorption step from more than 45 seconds to typically less than 10 seconds. Special zeolites have been developed to enable such a change to be made to the pressure swing adsorption process. Examples of such "fast cycling" pressure swing adsorption processes are given in U.S. Pat. No. 4 576 614 (corresponding to GB 2 091 121A) and U.S. Pat. No. 4 194 891. One of the consequences of adopting a "fast cycling" PSA process is that the velocity of gas flow through each adsorbent bed tends to be relatively high. In addition, the average particle size of the zeolite adsorbent tends to be smaller than that of the zeolites adapted for use in more conventional PSA processes in which each adsorption step takes in the order of a minute or more to perform. Both these factors lead to problems of localised fluidisation of the bed which in turn can cause attrition of the sieve and hence the formation of dust. Such dust particles are then carried out in the gas mixture and can become lodged in operating valves. There is therefore a decline in the performance of the adsorption bed in which severe cases may be sufficient to cause the process to fail to meet its design performance and also a risk of the dust particles hampering effective operation of valves associated with the adsorbent vessels.
In U.S. Pat. No. 4 576 614 there is disclosed a means for counteracting this tendency for the adsorbent bed to be fluidised. Pressure is exerted on the top of each bed by means of an upper grid or plate loaded by means of compression springs. Since adsorption beds in pressure swing adsorption processes tend to be generally columnar in shape, a relatively high compressive force is exerted by these plates on a relatively small surface area of sieve. Accordingly, there is a tendency for the sieve at the top of the bed to be crushed and as a result the sieve may be damaged.
The above description of the kind of problem that arises in a "fast cycling" pressure swing adsorption process is intended to be illustrative of the kind of problems that arise in operating beds of particulate material. Other problems that can arise include the tendency of the bed to "settle" over a period of time thereby giving rise to the bed having an uneven height with the result that the gas will try to flow more through some parts of the bed than others and hence the bed will not be efficiently utilised.